Registration of nuclear medicine images

ABSTRACT

A method of registering a plurality of functional images includes providing a plurality of functional images, providing a plurality of structural images, each one of which having a known positional relationship to at least one of said plurality of functional images, and finding a first mapping transformation between pairs of functional images based on the first mapping transformation and the positional transformation.

This application is a divisional application of U.S. application Ser.No. 08/454,871, filed on May 31, 1995, now abandoned, and is related toU.S. Application No. 08/571,429, now U.S. Pat. No. 5,871,013, filedcurrently herewith and which is also a division of the same application.

FIELD OF THE INVENTION

The present invention relates to the art of diagnostic imaging. Inparticular, the invention relates to nuclear imaging systemsincorporating simultaneous transmission and emission tomography ormulti-energy window tomography.

BACKGROUND OF THE INVENTION

SPECT (Single Photon Emission Computerized Tomography) is used to studythe three dimensional distribution of a radionuclide in a patient.Typically one or more radiopharmaceuticals are ingested or are injectedinto the patient. When radiopharmaceuticals are injected it is usuallyinto the patient's blood stream, to image the cardio-vascular system orto image specific organs which absorb the injected radiopharmaceuticals.One or more gamma or scintillation detectors are positioned near thepatient to record emitted radiation.

SPECT images are generally produced by:

-   -   (a) rotating the detector(s) around the patient in order to        record emissions from a plurality of directions; and    -   (b) transforming the recorded emissions, using methods well        known in the art, into a tomographical multi-slice image, a        three dimensional image or some other representation of the        distribution of the radiopharmaceutical injected into the        patient's body.

One problem with SPECT is that the tissues surrounding the organs beingimaged attenuate and scatter the radiation emitted by theradiopharmaceutical, distorting the resulting SPECT images. To solvethis problem, a SPTCT (Single Photon Transmission ComputerizedTomography) image of the region being imaged, is acquired,simultaneously with the SPECT image. The SPTCT image providesinformation regarding the attenuation and scattering characteristics ofthe region being imaged, so that the multi-view emission data can becorrected.

In order to acquire the simultaneous SPTCT image, a source of radiationis placed opposite the patient's body from the detectors(s) and rotatedwith the detector(s). Preferably, but not necessarily, the energy of theSPTCT source is different from that of the radiopharmaceutical so thatthe detector is able to easily differentiate the two radiations.

Since the emission image is acquired at the same time as thetransmission image, and the relative geometry of the SPTCT and SPECTsystems are known, the images are easily registered to one another.

The diagnostic method that uses SPECT and SPTCT simultaneously is knownas STET (Simultaneous Transmission and Emission Tomography). This methodis described in further detail in U.S. Pat. No. 5,210,421, thedisclosure of which is incorporated herein by reference.

One aspect of the present invention relates to the use of STET imagingtechniques for functional imaging. In this use, the resultant STET imageshows the metabolic activity of body tissue, since dead or damaged bodytissue absorbs the radiopharmaceutical at a different rate (or not atall) from healthy tissue. When used in this manner, the STET image showsthe functional activity of the body tissue, not its structural detail.

However, STET images have two drawbacks. First, as indicated above, theSTET image does not show much structural detail; therefore, it isdifficult to pinpoint where the imaged function is occurring in thepatient's body. Many diagnostic imaging methods, in modalities otherthan nuclear medicine, reveal almost exclusively structure and notfunction, therefore, it is hard to compare STET images with other typesof diagnostic images. Second, a common methodology, especially incardiac examination, is to acquire a STET image shortly after injectionof the radio pharmaceutical and to acquire another STET image of thesame region after a certain period of time. By comparing these two (ormore) images, it is possible to learn still more about the function ofthe tissue studied, such as the speed at which different portions oftissue absorb aid metabolize the radiopharmaceutical. However, if thetwo STET images are too different, it is not possible to closely comparethem because the operator can not match the different parts of theimages to each other.

SUMMARY OF THE INVENTION

The present invention contemplates a method for registering STET imagesand other functional images to images of other modalities, and formatching two STET images taken at different times of the same bodyregion, thereby solving the above mentioned problems.

In accordance with one preferred embodiment of the present invention, amethod for matching two STET images acquired at different times uses theSPTCT data in order to identify structure in the patient's body. Whentwo STET images are to be compared, the two respective SPTCT images areregistered, preferably, using a correlation method or another knownimage matching method. Since the STET image is registered to its SPTCTimage, registering the two SPTCT images automatically registers the twoSTET images.

In accordance with another preferred embodiment of the presentinvention, a method for registering a STET image and a structuraldiagnostic image (such as an MRI, ultrasound or X-ray CT image) uses theSPTCT data in order to identify structure in the patient's body. Whenthe STET image is to be registered to the structural diagnostic image,the structural SPTCT image and the structural diagnostic image areregistered. This registration is preferably accomplished through thechoosing and comparing of prominent body structures, such as theskeleton, organs or body outlines. Once this matching is accomplished, amapping between the images can be defined, based on the mapping betweenthe prominent body structures chosen. This mapping is used to transformone image so that it can be superimposed over the other image.

Alternatively, prominent body markings on the SPTCT image are saved asfiduciary marks with the STET image. These marks are used to match theSTET image to another structural image.

In accordance with yet another preferred embodiment of the presentinvention, a method for registering a first SPECT image to a structuraldiagnostic image uses a second SPECT image to serve as a structuralimage. Two SPECT images are acquired of the studied region, the firstimage is acquired using a first radiopharmaceutical, which is selectedso that the resultant SPECT image shows the desired function. The secondSPECT image is acquired using a second radiopharmaceutical, which isselected so that the resultant image shows some structure, such asoutlines of organs which can be used to register the second SPECT imageto another structural image. Alternatively, parameters other than theradiopharmaceutical are varied in order to generate the different SPECTimages.

Matching between the second SPECT image and the structural diagnosticimage is accomplished through the choosing and comparing of prominentbody structure shown in both images. Preferably, the two SPECT imagesare acquired simultaneously using a dual isotope gamma camera, so thatthey are automatically registered.

A mapping between the first SPECT image and the structural diagnosticimage is then created based on the inherent registration between the twoSPECT images and the matching between the second SPECT image and thestructural diagnostic image. It should be noted that this preferredembodiment does not require a STET device, a SPECT device is sufficient.

In a simple situation, the size and shape of the images is not affectedand only translation and/or rotation is required. Where scaling isrequired, one of the images is scaled in accordance with the correlationof a plurality of chosen structural features or of the images as awhole. In one embodiment of the invention, warping and other complexcorrections can be applied to improve the match between the images.

The term “structural image” as used herein means an image that is usedto compare structures. The term “functional image” as used herein meansa functional image that is not used to determine registration. As can beappreciated, functional images may show structure and a substantialamount of structure in structural images may be caused by functionality.

Preferably for many types of studies, the acquisition of SPECT, SPTCTand STET images is synchronized to the cardiac rhythm, the respiratoryrhythm or other body motions by gating. In such gated images dataacquired during the imaging process is binned (or windowed) according toa gating signal derived from the body rhythm.

Thus, in a preferred embodiment of the invention, image acquisition isgated to body rhythms and motions. Preferably, the structural images arealso synchronized in the same manner. For example, gated CT images areused as structural images instead of regular CT images when the STETimages are gated. An advantage of combining STET imaging with gating isthe ability to correct binned data for patient motion during dataacquisition by realignment based on the registration of the images. Thiscorrects for smearing otherwise produced by patient motion.Additionally, data from separate bins is more easily combined.

Another advantage is the ability to correct organ motion caused by thegated rhythm, by applying a geometric transformation to data acquiredbased on the phase of the gated rhythm. Yet another advantage is theability to register transmission images to emission images even whenthey are not acquired simultaneously. A transmission image of a patientwhich is gated to body rhythms can be automatically registered to itscorresponding gated emission image, since most of the misalignmentbetween the two images is caused by body rhythms which are, in general,repetitive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial, simplified schematic view of a slice of the humanbody in the chest region, showing the heart, ribs and a portion offunctioning heart tissue;

FIG. 2A is a simplified schematic of a SPTCT scan of the body slice fromFIG. 1;

FIG. 2B is a simplified schematic of a STET image of the body sliceshown in FIG. 1;

FIG. 2C is a simplified schematic of a STET image of the body sliceshown in FIG. 1, acquired at a different time from FIG. 2B;

FIG. 3 is a simplified schematic X-ray CT image of the body slice shownin FIG. 1.

FIG. 4A is a simplified correlated STET image created by aligning andsuperimposing the STET images from FIG. 2B and FIG. 2C;

FIG. 4B is a superposition image created from the functional STET imagein FIG. 2B and the structural image from FIG. 3;

FIG. 5 is a simplified schematic STET image with fiduciary marks foraiding in correlation with structural images such as X-ray CT scans; and

FIG. 6 is a simplified block diagram of a STET system includingequipment for cardiac and respiratory gating.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention does not require the use of any specific STETdevice, and for most devices the invention can be practiced by changesand/or additions in image processing and registration. In addition, itis possible to use the present invention with NON-STET devices, providedthat the SPECT and SPTCT images can be registered to each other.

FIG. 1 in U.S. Pat. No. 5,210,421 shows a typical STET camera assemblywhich is used for acquiring STET images.

The process for acquiring these images typically includes:

-   -   (a) placing a patient on a couch, so that the part to be studied        will be in an examination area;    -   (b) injecting a radiopharmaceutical into the patient;    -   (c) acquiring pairs of SPTCT and SPECT images using one or more        detectors;    -   (d) rotating the detector(s) around the examination area, in        order to acquire a plurality of image pairs;    -   (e) transforming the plurality of image pairs into a multi-slice        tomographical STET image, a three dimensional STET image or        another representation of STET data, the SPTCT images being        employed to correct the attenuation and scattering artifacts in        the SPECT images to produce the STET images;    -   (f) optionally, after an attending physician examines this        image, the patient is sent to rest and/or exercise and/or        reinjection;    -   (g) after a period of rest or exercise, the image acquisition        process is typically repeated, with the patient placed in as        nearly as possible the same position as during the previous        study, so as to facilitate comparing the new images with the old        ones.

Preferably for many types of studies, the acquisition of SPECT, SPTCTand STET images is synchronized to the cardiac rhythm, the respiratoryrhythm or other body motions by gating. In such gated images dataacquired during the imaging process is binned (or windowed) according toa gating signal derived from the body rhythm.

The following discussion refers to a section of the patient's body beingimaged, shown in FIG. 1. FIG. 1 is simplified to include only a heart 1including a functionally active area 2 of the heart, ribs 8 and abackbone 3. In order to simplify the discussion, only one slice isshown, even though the STET image is three dimensional. Application ofthe invention to three dimensions and choosing the correct slices isdescribed below.

FIG. 2B shows a STET image 6 of the body slice shown in FIG. 1, such aswould be acquired in a heart study. In such studies, most of theradiopharmaceutical is concentrated in the blood or in soft tissues andspecific organs such as the heart and liver, so that the acquired STETimage 6 shows mostly portions of target organs and a fuzzy outline 9 ofthe patient's body. FIG. 2C shows a later STET image 6′ of the sameregion in the same patient. With the passage of time, theradiopharmaceutical is absorbed and metabolized by the body tissues, andthe STET image changes, as can be seen by comparing image 6 with image6′. In FIG. 2C a functionally active area 2′ is imaged which is largerthan area 2.

FIG. 2B and FIG. 2C are STET images 6 and 6′ of the region shown in FIG1. The images 6 and 6′ show functionally active areas 2 and 2′respectively but not bones such as the ribs 8 or even the non-activeareas of heart 1. FIG. 2A shows a very simplified SPTCT image 7 which isa structural image, much like a standard X-ray CT, except for poorerresolution and lower organ definition ability. The SPTCT image 7, showsheart 1, ribs 8 and even backbone 3, but does not specificallydifferentiate the functionally active areas of the heart.

In the later STET image 6′, of FIG. 2C, there are significant changesfrom the earlier STET image 6, of FIG. 2B, making it difficult, if notimpossible, to match correctly functioning area 2 in image 6 withfunctioning area 2′ in image 6′. In addition, it is difficult toidentify correctly the structural areas which are functioning asrevealed by the radiopharmaceutical.

A second SPTCT image is acquired simultaneously with image 6′. The SPTCTimages acquired with images 6 and 6′ are very similar, since thepatient's body structure does not change much between the images, andthe continuing diffusion of the radiopharmaceutical which plays acrucial part in images 6 and 6′ does not play a part in SPTCT imaging.Two types of differences between the two SPTCT images are caused by:

-   -   (a) changes due to patient movement caused, for example, by        breathing, and    -   (b) changes due to different placement of the patient on the        examination table.

Since the respective emission and transmission images are acquired withthe same known system geometry, the mapping of the emission image to itsrespective transmission image is also known, so the two respectiveimages can be considered registered to each other. The followingdiscussion assumes that any necessary registration between the tworespective images has been performed.

A preferred embodiment of the invention uses the following process inorder to transform a SPTCT structural image, which has an associatedregistered STET image, so that it is registered to a structural image:

-   -   (a) marking prominent body structures in the two structural        images;    -   (b) correlating the prominent structures between the structural        images;    -   (c) determining a transformation between the two structural        images, based on the correlation between the structures; and    -   (d) transforming the SPTCT image in accordance with the        transformation found in (c).

The transformation will have a degree of complexity appropriate to theimages being aligned, and may include:

-   -   (i) simple alignment of the images;    -   (ii) scaling of one of the images; and    -   (iii) warping one of the images.

The functional STET image associated with the SPTCT image is transformedusing the same transformation as that used for the SPTCT image.

In a preferred embodiment of the invention, registering of two STETimages 6 and 6′ is achieved by registering the two respective associatedSPTCT images using the above described method. The registration of STETimages 6 and 6′ follows automatically.

In an additional preferred embodiment of the invention a STET image 6 isto be registered to a structural image such as a X-ray CT image, a MRIimage or an ultrasound image. FIG. 3 shows a CT image 70, such as is tobe registered to STET image 6. The registration is preformed by usingthe above described process to register SPTCT image 7, that isassociated with STET image 6, to CT image 70. The registration of STETimage 6 to CT image 70 follows automatically, using the sametransformation used to register the two structural images.

In yet another preferred embodiment of the invention, a SPECT image isregistered to a structural image, such as an X-ray CT image, using asecond SPECT image as a structural image instead of using a SPTCT image.

A SPECT device is used to simultaneously acquire two images, with oneimage showing enough structure to be used as a structural image. The twoimages are acquired using a dual isotope gamma camera and a differentradiopharmaceutical for each image. Since the functional and thestructural SPECT images are automatically registered, registering thestructural SPECT image with the X-ray CT image or other structural imageautomatically registers the functional SPECT image with the X-ray CTimage or other structural image. Accordingly, the registration betweenthe structural SPECT image and the structural image is performed byusing the above described registration process. The registration of thefunctional SPECT image to the structural image follows automatically,using the same transformation used to register the two structuralimages.

For example, to detect and locate malignant liver lesions, two SPECTimages and one CT image are acquired of the liver. A first SPECT image,which is acquired using FDG as a radiopharmaceutical, highlights onlymalignant tumors and shows little body structure. A second SPECT image,acquired simultaneously using intravenously injected Tc99m colloid,clearly shows the anatomic boundaries of the liver and lesions. A CTimages of the liver and surrounding tissue also clearly shows theanatomic boundaries of the liver and lesions. Therefore, the CT image(the structural image) is registered to the second SPECT image (thestructural SPECT image) using the registration process described herein.Consequently, the first SPECT image is registered to the CT image(because the two SPECT images are acquired simultaneously and,therefore, automatically registered to each other) so that the malignantlesions can be pointed out on the CT image.

Typically a three dimensional image is acquired and processed as aseries of two dimensional slices. In order to properly register slicesof three dimensional images, as described above, slice pairs that havethe same location along the patient's longitudinal (Z) axis must bechosen.

In the case of matching two STET images, corresponding slices from thetwo SPTCT images must be chosen. Two preferred methods for matchingslices are:

-   -   the operator chooses the appropriate slices, based on his/her        understanding of the images and his/her knowledge of human        anatomy; and    -   (ii) since the image modality is the same for both SPTCT images,        a computer can search for the closest matching slice pair using        a correlation algorithm.

Once the closest matching slices are found, the process continues asdescribed above. Alternatively, using image matching techniques known inthe art of image processing, the two SPTCT images can be matched in theaxial direction with a precision higher than the width of a slice. Sincethe STET image is a true three dimensional image, one of the two imagescan be “re-sliced”, so that the image slices of one STET image areexactly aligned to the slices of the other STET image.

In the case of registering a STET image to a X-ray CT image, thepreferred way to find the correct matching CT and SPTCT slices is tohave the physician choose the slice pair, based on his understanding ofthe images and his knowledge of human anatomy. Once the closest matchingslices are found, the STET image can be re-sliced so that the STET imageslices fall on boundaries of the CT slices. For images derived fromdifferent modalities, the Z scale may be different. A slice scale factormay be derived based on matching a plurality of structural features indifferent slices.

In an additional preferred embodiment of the invention, steps (a) and(b) of the registration process are replaced by a single step ofcorrelating the two images as a whole. Additionally, three dimensionalimages may also be correlated as wholes, without first slicing them andcorrelating the slices.

In order to facilitate manual finding and matching or marking ofprominent body structures between images, it is useful to display theimages as three-dimensional images on a computer screen and mark theprominent structure on the three-dimensional images, so that theattending doctor will not have to work directly with image slices.

Once the transformation between the two images is known, many imageprocessing techniques are applicable, for example: image subtraction,rapid flipping of too or more images, superpositioning of outlines ofthe active areas from one STET images on another STET image or on a CTimage and pseudo coloring of different areas. FIG. 4A shows thesuperpositioning of the outline of an active area from the STET image 6on the STET image 6′. FIG. 4B shows the superpositioning of the outlineof the active area from the STET image 6 on the CT image 70.

In addition, the present Invention enables simultaneous processing andviewing of several images which are registered to each other using themethods described herein. For example, two images are displayed side byside on a computer screen, a portion of one image is marked off andradiation emitted by that portion is computed. The radiation emitted bythe matching portion of the other image is calculated and displayedautomatically by the computer.

In general, the correlation algorithms used for matching images andslices, between and within modalities and the subsequently derivedtransformations are any of a variety of methods known in the art ofimage registration. The following image registration methods are usefulin carrying out preferred embodiments of the invention.

1. Landmark matching. Corresponding anatomical or external markers areidentified in the sets of data to be matched. A minimum root meanssquare alignment transformation is then calculated to align one set ofmarkers with the other set. Preferably, the markers are identified by anoperator.

2. Surface matching. The surface representations of two data sets arecorrelated by finding the transformation which yields the minimum rootmean square distance between the two surfaces. This method is describedin “Accurate Three-Dimensional Registration of CT, PET and/or MR Imagesof the Brain”, by Pelizzari C. A., et al., Journal of Computer AssistedTomography, volume 13, 1989.

3. Volume matching. The two data sets are correlated by finding thetransformation which yields the maximum cross correlation value betweenthe sets. This method is described in “MRI-PET Registration withAutomated Algorithm”, by Woods R. P., et al., Journal of ComputerAssisted Tomography, volume 17, 1993.

4. Spatial parameters matching. The two data sets are correlated bymatching spatial parameters such as the moments of the data sets. Themoments can be matched by finding the principle axis for which theyattain their minimal value. This method is described in “The principleAxes Transformation—a Method for Image Registration”, by Alpert N. M.,et al., Journal of Nuclear Medicine, volume 31, 1990.

5. Invariant geodesic lines and points matching. The data sets areanalyzed using a differential analysis of their surfaces discreterepresentation, yielding lines and points which correspond to localmaxima and/or minima of surface curvature. A global affinetransformation is then found that delivers the best matching of thecorresponding lines and points from the two data sets. This method isdescribed in “The External Mesh and the Understanding of 3D Surfaces”,research report number 1901 from the Institute National de Recherche enInformatique et en Automatique (INRIA), May 1993, and “New FeaturePoints Based on Geometrical Invariants for 3D Image Registration”,research report number 2149 from the INRIA, both by Jean-PhillipeThirion.

In an additional preferred embodiment of the invention, fiduciary marksmay be added to the STET image by first adding fiduciary marks to astructural image that is registered to the STET image, and thentransforming those marks to the STET image. Additionally, these marksmay be added from a template once the transformation is known. FIG. 5shows a STET image with fiduciary marks thereon.

In a further preferred embodiment of the invention, image acquisition isgated to body rhythms and motions. Preferably, the structural images arealso synchronized in the same manner. For example, gated CT images areused as structural images instead of regular CT images when the STETimages are gated. An advantage of combining STET imaging with gating isthe ability to correct binned data for patient motion during dataacquisition by realignment based on registration of the images. Thiscorrects for smearing otherwise produced by patient motion and enablesthe use of longer acquisition times. Additionally, data from separatebins is more easily combined.

Another advantage is the ability to correct organ motion caused by thegated rhythm, by applying a geometric transformation to data acquiredbased on this phase of the gated rhythm. Yet another advantage is theability to register transmission images to emission images even whenthey are not acquired simultaneously. A transmission image of a patientwhich is gated to body rhythms can be automatically registered to itcorresponding gated emission image, since most of the misalignmentbetween the two images is caused by body rhythms which are, in general,repetitive.

FIG. 6 indicates in simplified block diagram form a STET system 21equipped to accomplish either cardiac or respiratory gating or both.System 21 generally comprises a detector 22 for detecting radiation. Theradiation can tie emanating from a patient 23 or from a radiation source24, typically comprising a radioisotope material. When source 24 is aradioisotope, detector 22 is preferably an Anger type camera.

The output of detector 22 is processed by a signal processor 26.Processor 26 determines the location and energy of photons strikingdetectors 22.

The output of signal processor 26 is further processed by imageprocessor 27 to provide image data using a memory 28. The processedimages are shown on display 29.

Gating controls are provided for system 21. More particularly,respiratory gating uses a position sensor 31 which senses the thoraxposition of patient 23 during the STET process. The sensed displacementis operated on to provide windows or bins using a displacement detector32. A position gate signal unit 33 provides gating signals to signalprocessor 26 based on the thorax position determined by detector 32. Thecardiac gating system senses the heart beat with a sensor 36. The R-waveis detected by a wave detector 37. A cardiac gating signal is providedto signal processor 26 by a wage gate signal unit 38 responsive todetection of the R-wave by detector 37. U.S. Pat. No. 4,617,938, thedisclosure of which is incorporated herein by reference, describes agating system.

STET system 21 is shown to be under the control of a controller 41 whichsupplies the appropriate control and timing signals.

The present invention was described in the context of nuclear medicineimaging. However, the present invention is applicable to other types ofimaging systems, provided that functional images (as described herein)have structural images that are registered to them where needed.Additionally, structural images of modalities other than X-ray CT, MRI,ultra sound and SPECT can be registered to nuclear medicine images byutilizing the present invention.

It will be appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed herein. Rather, the scope of the present invention is definedonly by the claims which follow:

1. A method of registering a functional image to a structural diagnosticimage comprising: providing a functional image; providing a structuralimage having a known mapping transformation to said functional image,said known mapping transformation being determined based on theapparatus and methods used to acquire the structural and functionalimage having the known mapping transformation; determining referencepositions on said structural image; and marking said functional image atpoints associated with the reference positions using the known mappingtransformation.
 2. A method according to claim 1, wherein marking isdone with fiduciary marks provided from a template.
 3. A methodaccording to claim 1, comprising matching to a different image usingfiduciary markings that are registered to said functional image.
 4. Amethod according to claim 1, wherein: said providing a functional imagecomprises acquiring a functional image based on a first set of acquireddata; and said providing a structural image comprises acquiring astructural image based on a second set of acquired data different fromthe first set of acquired data.